Date of Award
Doctor of Philosophy (PhD)
Chair and Committee
This thesis reports the microanalysis of sixteen presolar SiC grains that are thought to have condensed in several different types of astrophysical environments, including asymptotic giant branch: AGB) stars, J stars, born-again AGB stars, and Type II supernovae: SNe). The isotopic compositions and microstructural properties of a large number of presolar grains have previously been measured, but the two techniques have been combined in the analysis of only a handful of presolar grains. Detailed structural and chemical microanalytical studies, when combined with isotopic measurements, provide the most complete constraints on presolar grain formation and growth. With the transmission electron microscope: TEM), I analyzed presolar SiC grains to determine the crystal size and structure of isotopically characterized SiC grains, as well as the elemental composition of the grains. I also obtained this information for internal subgrains found within the SiC grains. My combined analysis of these grains helped to place constraints on the growth conditions during grain condensation, revealed differences between the physical conditions in different types of stellar sources, suggested the likely origin of these grains, and identified new types of presolar grains. The types of internal subgrains I observed in the presolar SiC grains in this study varied according to the stellar source of the grains. During TEM analysis, I discovered two new types of presolar grains that are present as subgrains within the SiC grains. One of these types was determined to be iron-nickel silicide [(Fe,Ni)mSin], which is found in SiC grains that originated in SNe. Although silicides are predicted stable SN condensates, they have not been previously observed in presolar grains. The other new type of presolar grain is oldhamite: CaS), found in a SiC grain that most likely condensed in the envelope of a J star. Oldhamite is one of the lowest temperature minerals that are predicted to condense with SiC and its presence, along with the observation of other low temperature condensates in the same SiC grain, indicates that thermochemical equilibrium must have been maintained over a wide range of temperatures during SiC grain condensation. I observed that SiC crystal domain size varies with stellar source, presumably as a result of the very different growth conditions that exist in the diverse circumstellar environments in which each type of SiC is thought to have condensed. However, I did not observe the crystal structure of the SiC grains to vary with stellar origin. In addition to the TEM analysis of presolar SiC grains, I also compiled a database of all of the existing isotopic data on presolar grains, which is accessible via a website. This is both the first time that data on the more than twelve thousand presolar grains reported in the literature have been organized in a single place, and that a significant compilation of presolar grain data has been made available to the entire astrophysical community. I designed the database as a tool to be utilized by both experimentalists and theorists, allowing them to readily compare their data or models to previous measurements and to serve as a simple, quick reference library of all the available presolar grain papers from which I obtained the original data.
Hynes, Kathryn, "Microanalytical Investigations of Presolar SiC Grains as Probes of Condensation Conditions in Astrophysical Environments" (2010). All Theses and Dissertations (ETDs). 865.